1. Field of the Invention
The present invention relates to a switch gear, and more particularly to a switch gear comprising a vacuum circuit breaker, vacuum disconnector or similar vacuum switch used for interruption and continuity or switching in electric power and reception and/or distribution systems.
2. Description of the Related Art
FIG. 1 illustrates an example of the configuration of a typical switch gear as used in electric power and reception and/or distribution systems. In FIG. 1, a receptacle 101 is surrounded with mild steel plating and divided internally with a partition 103. A circuit breaker chamber 105 at the front houses a circuit breaker 109 fitted with a vacuum valve 107, while a bus-line chamber 111 at the rear is equipped with identical disconnectors 113 and 114 corresponding to upper and lower main circuits on the circuit breaker 109 side. The upper disconnector 113 side is connected to a bus-line 117 which is fixed to a supporting insulator 115, and is thence connected to a neighbouring board. The lower disconnector 114 side is connected to a cable head 121 which is fed by an electric power cable 119. These pieces of apparatus are mutually connected by means of a connecting conductor 123. In the partition 103 which separates the source side from the load side is an aperture not illustrated in the drawing is an insulating spacer 125 formed by molding the principal circuit conductor in an insulating layer, and the principal circuit is connected to the mutual partition of the chambers 105 and 111. These chambers 105 and 111 are charged with an insulating medium such as SF6 gas.
SF6 gas is characterized among other qualities by being colorless, harmless and inert. At atmospheric pressure it has 2-3 times the dielectric strength of air. A gear switch charged with a gas of this sort ensures a stable supply of electric power.
In a configuration of this sort, SF6 gas has a higher degree of dielectric strength than air, and consequently allows the switch gear to be made more compact as described for instance in Japanese Laid-Open Patent Application S60[19851]-210107. However, at the conference on the prevention of global warming held in Kyôto in December 1997 it was revealed that its contribution to global warming is some 23000 times that of carbon dioxide gas, and it was determined that every effort should be taken to prevent it leaking or being released into the atmosphere.
This means that it is vital to ensure that the welded sections of the steel plating which seals the receptacle 101 is airtight, and that there is no gas leakage from the O-ring which is used in the gas/air section of the cable head 121. It is also necessary to collect the gas in a gas collector prior to opening the receptacle 101 for the purpose of inspecting the interior thereof. It goes without saying that these measures have been taken in apparatus which operates by the conventional method, but they become even more important and require countermeasures in order to guarantee safety.
Such measures become unnecessary if SF6 gas is not used, but the fact remains that there is no better gaseous insulation medium. For example, it is possible to use air, but this results in inferior dielectric strength. This must be resolved by increasing the insulation distance, which results in an overall enlargement of the apparatus. On the other hand, normal insulation within air is affected by dust and moisture, and the creepage distance must be increased in view of this. Such considerations run contrary to recent trends towards greater compactness in devices of this sort.
As a result, attempts have been made as in Japanese Laid-Open Patent Application H9[1997]-013027 to avoid the use of SF6 gas by adopting a configuration whereby a vacuum valve is molded directly as a solid insulator. However, this method is flawed in that it also requires the other component parts of the switch gear to be treated by molding or otherwise to insulate them, in addition to which the connections need to be treated after assembly so that the conductor does not leak. This involves a considerable number of man hours and extra cost.